Wet media mills, such as the ones described in U.S. Pat. No. 5,797,550 issued to Woodall, et al, and U.S. Pat. No. 4,848,676 issued to Stehr, are generally used to mill or grind relatively large quantities of materials. These rather large media mills are not generally suitable for grinding small or minute quantities. U.S. Pat. No. 5,593,097 issued to Corbin recognizes the need for milling small quantities, as small as 0.25 grams, to a size less than 0.5 micron to about 0.05 micron in terms of average diameter in about 60 minutes.
The media mill described in the Corbin patent comprises a vertically oriented open top vessel, a vertically extending agitator with pegs, a motor for rotating the agitator, and a controller for controlling the rotational speed. The vessel is a cylindrical centrifuge or test tube formed of a glass, plastic, stainless steel, or other suitable material having an inner diameter of between 10 to 20 mm. The media suitable is described as any non-contaminating, wear resistant material, sized between about 0.17 mm to 1 mm in diameter.
The particulates to be ground and the grinding media are suspended in a dispersion and poured into the vessel. The agitator, with the peg end inserted in the vessel, is spun. The Corbin patent also discloses that the pegs should extend to within between about 1–3 mm of the sides of the vessel to provide the milling desired in the shortest possible time without damaging the materials and producing excessive heat. To avoid splattering created by vortexing of the material during mixing, the top peg of the mixer is positioned even with the top of the dispersion. No seal or cover is deemed needed during mixing or agitation if this practice is followed.
The Corbin patent also discloses that its micro media can be useful for forming medicinal compounds, food additives, catalysts, pigments, and scents. Medicinal or pharmaceutical compounds can be expensive and require much experimentation, with different sizes and quantities. The Corbin patent discloses that the preferred media for medicinal compounds are zirconium oxide and glass. Moreover, pharmaceutical compounds are often heat sensitive, and thus must be maintained at certain temperatures. In this respect, the Corbin patent discloses using a temperature control bath around the vessel.
In the media mill of the type described in the Corbin patent, even if the vessel is filled to the top peg, however, the rotating agitator in the dispersion creates a vortex, which undesirably draws air into the dispersion and foams the dispersion. Moreover, the open top configuration draws in contamination, making the mill unsuitable for pharmaceutical products. The temperature-controlled bath could spill into the open top container and further contaminate the product.
There is a need for a micro or small-scale media mill that avoids these problems. The present invention is believed to meet this need.